1. Field of the Invention
This invention relates generally to a cryostat with a liquefaction refrigerator, and, more particularly, to a cryostat with a liquefaction refrigerator which contains a superconducting magnet, and can easily condense evaporation gas of liquefied gas, with the cryostat being suited for use in a supermagnetic resonance image pickup device or the like.
2. Description of the Prior Art
In a conventional cryostat with a liquefaction refrigerator, particularly, a nuclear magnetic resonance device (hereinafter referred to as "MRI") used in a medical field for the diagnosis of a human body, as disclosed in Japanese Patent Unexamined Publication No. 62-299005, a superconducting magnet (hereinafter referred to as "magnet") is employed for obtaining a uniform high magnetic field. Therefore, a vacuum-insulated cryostat for containing the magnet in a cooling condition is needed.
The magnet is cooled by dipping it in a cryogenic cooling medium, e.g. liquid helium, so that the magnet can be maintained in a superconducting condition. The liquid helium is gradually evaporated by the heat intruding from an ordinary-temperature vacuum vessel of the cryostat into the cryostat, and therefore it is necessary to periodically replenish or pour liquid helium. If the amount of evaporation of the liquid helium is reduced, and if it is re-liquefied in the cryostat, the pouring of the liquid hellium is unnecessary, and the operating cost is greatly reduced.
Therefore, in the prior art, a liquid refrigerator includes a refrigerator for cooling a thermal shield plate within the cryostat, and a liquefaction cooler fixedly mounted in the vacuum within the cryostat so as to cool the liquid helium to a liquefaction temperature, utilizing the cold of the refrigerator.
Also, in the prior art, helium gas, compressed by a compressor unit of the oil lubricating type mounted within an ordinary-temperature room, is supplied to the liquefaction cooler B, and the supplied helium gas contains impurities, such as oil components, N.sub.2, O.sub.2 gas and CO.sub.2 gas, whose concentration is several ppm.
The refrigerator is mounted within a sleeve fixedly mounted on the thermal shield plate within the cryostat, and a heat exchange between a cold station of the refrigerator and the wall of the sleeve is effected through helium gas filled and sealed in the sleeve.
The refrigerator and the liquefaction cooler both receive operating gas (e.g. high-pressure helium gas) from the compressor mounted within the ordinary-temperature portion, and therefore impurities, such as oil, H.sub.2 O, CO.sub.2, N.sub.2 and H.sub.2 gas, are accumulated in cooling portions of the refrigerator and the liquefaction cooler, and they must be overhauled periodically.
Those constituent elements of the refrigerator which require the overhaul are a displacer (washing of the deposited cooling material) of a reciprocating-type expander, such as Gifford-McMahon and Solbey cycle, a seal ring (exchange) provided at the cooling portion, and etc. Those constituent elements of the liquefaction cooler which require the overhaul are a heat exchanger (washing of its interior), a Joule Tomson valve (washing of its interior), and etc. In the above prior art construction, the displacer of the refrigerator can be easily withdrawn, with its cylinder portion left in the cryostat, and its overhaul can be easily accomplished; however, since the liquefaction cooler is fixedly mounted within the liquefaction cryostat, the cooler must be removed after the vacuum in the cryostat is once returned to an atmospheric pressure.
However, to carry out this operation, a liquid helium vessel must be once returned to an ordinary temperature, and the removal of the liquid helium from this vessel, the heating, the re-pouring of liquid helium after the overhaul and the re-cooling are needed, results in an increased cost for the overhaul.
The cycle of overhaul of the liquefaction cooler is determined by the concentration of the impurities in the operating fluid of the liquefaction cooler. The higher impurity concentration, the sooner the overhaul cycle, and the overhaul cost is high, thereby resulting in an increase in the overall operating cost.
The heat exchange between the cold station of the refrigerator and the wall of the sleeve is carried out by the thermal conduction of the stationary helium gas filled therebetween. Therefore, the thermal resistance of the helium gas layer is large, so that the temperature difference between the cold station of the refrigerator and the sleeve wall is large. As a result, the amount of evaporation of the liquid helium is increased due to the insufficient cooling of the thermal shield plate, and also the liquid surface of the liquid helium in the liquid helium vessel is lowered due to the insufficient cooling of the liquefaction cooler thermally integrated with the sleeve wall. As a result, it becomes necessary to re-pour the evaporation gas of the liquid helium gas, thus increasing the operating cost, and also the superconducting magnet is exposed to the liquid surface, so that its superconducting condition is quenched because of its temperature rise. Thus, the reliability of the cryostat is lowered.